It’s now possible to export your Facebook social graph. Your real social graph, complete with your friends’ emails, which means you can leave Facebook and import your social network into a rival site.

But only if your friends opt into it. And they won’t.

Facebook has quietly added a new feature to its Account Settings page, giving users the option to “Allow friends to include my email address in Download Your Information“. Check it, and the next time your friend decides to download an archive of their Facebook data, they’ll get your email address too so they can stay in touch.

It’s an ingenious move on Facebook’s part that will help undermine arguments that it is holding users’ data hostage, while simultaneously ensuring that it keeps a firm grip on its massively valuable social graph.

There’s a lot of history involved here, so let’s take a look back.

‘Download Your Information‘ is a feature that launched last fall, and it’s exactly what it sounds like: click it, and within a few hours you’ll receive an archive of your photos, status updates, and list of friends. Up until now that list of friends has been quite literal — it’s a list of your friends’ names, with no accompanying email address or contact information. Which means it’s effectively useless to other services that would love nothing better than to import your social graph. Like, say, Google.

Last fall, Google and Facebook engaged in a very public slap fight over contact data. Google struck first, blocking Facebook from importing users’ Google address books because Facebook wasn’t letting contact data flow the other direction. Which meant that Google would have a much harder time rebuilding the social graph when it launched what eventually became Google+.

Facebook’s argument was that your friends shouldn’t necessarily have the right to import your email address into another service. Which didn’t really make sense, because Facebook users can already export all of their friends’ email addresses to Yahoo Mail (and only Yahoo Mail). Then again, Google’s argument that users should be able to export their friends’ email addresses didn’t really make sense, because it previously made a nearly an identical case about data ownership when it blocked Facebook’s ability to import data from Orkut.

Anyway, hypocrisy aside, the underlying motivations for both companies are obvious enough. Google wants Facebook’s social graph (in part because Facebook’s already sucked so much data out of Google Contacts) and Facebook doesn’t want to give it to them.

Which brings us back to today’s launch. This small checkbox is perfectly in line with Facebook’s argument that users should have control over their data. And when people (or Google) complain that they can’t export their friends’ email addresses, Facebook can say it’s not because it’s holding them hostage — it’s that users simply aren’t interested in sharing that data.

But, again, Facebook knows full well that most people aren’t going to go digging through settings to check that box. Many of them probably don’t know the ‘Download Your Information’ option exists, and they certainly haven’t put much thought into whether their friends should be able to download their email address as part of that archive. The box isn’t checked, and it’ll stay that way.

Also worth noting: Facebook hasn’t been shy about automatically opting users into new features by default (Instant Personalization, anyone?), and yet it’s taken the ‘safe’ route here by making it opt-in. I’m sure Facebook would argue that they’ve learned from their past mistakes. Or maybe it just has absolutely no incentive to help people export their social graph.